Method for heating up or keeping warm the flow paths of a process plant

ABSTRACT

The invention relates to a method for heating up or keeping warm the flow paths of a process plant, wherein a stream of fluid provided for heating up or keeping warm the flow paths is guided in a circuit which comprises a heating device ( 8 ) for warming up the stream of fluid. According to the invention, to equalize the pressure loss occurring in the circuit, the stream of fluid is supplied as a suction medium to a jet pump ( 5 ) arranged in the circuit and is exposed there to a pumping medium supplied at a higher pressure.

The invention relates to a method of heating or keeping warm the flow passages of a process plant, the flow passages being heated or kept warm by a fluid made to flow in a closed loop that has a heater for warming the circulating fluid. The present invention relates to heat input into the process plant outside the usual process operation, in which usually a process fluid flows into the plant and is there subjected to a chemical and/or physical treatment or conversion and then immediately leaves the plant. In the process plant constituents of the process fluid can be in particular subjected to separation, cleaning and/or catalytic conversion.

A number of process plants, in particular plants that contain reactors filled with catalytic material, must be brought to a certain minimum temperature before the actual intended normal operation in order, for example to adjust the necessary reaction temperature for the intended operation or in order to remove water or other components from the process plant, for example from the bulk filling of a reactor or the walls of the flow passages. It can also be necessary thereby to comply with certain heating-up rates, whereby over the duration of the heating process different heating rates are also to be provided depending on the design of the process plant.

The heating of process plants in practice usually takes place by direct heat exchange, flowing a heated fluid through the passages of the process plant. As a rule, inert gases, such as, for example, nitrogen or carbon dioxide or also water vapor are used for the heat exchange. Flowing the heated fluid through the process plant leads to a loss in pressure and thus causes a certain pressure gradient. With the known measures, an inert gas can be held in a sufficient amount in a reservoir, whereby the heating takes place by feeding the heated inert gas flow out of the reservoir. The disadvantage with this is that large quantities of inert gas have to be held in a correspondingly large reservoir and that the large amount of inert gas necessary for heating leads to high operating costs.

It is further known from practice to heat up the flow passages of a process plant by a closed inert-gas loop. Starting from a method with the features described above, to overcome the loss of pressure occurring in the process plant a compressor or a is circulating blower is then necessary, the entire investment costs of the process plant being considerably increased by this additional machinery that is not used in the normal operation of the process plant.

Against this background the object of the invention is to provide a method of heating or keeping warm the flow passages of a process plant, which with respect to operating and investment makes possible a reduction of costs compared to the known measures.

The object of the invention is attained according to the method of claim 1. According to the invention, to compensate for the pressure loss occurring in the closed loop the fluid flow is fed as an inlet fluid to a jet pump provided in the closed loop and is there acted on by a motive fluid fed at a higher pressure separately from the process fluid. In contrast to the completely closed loop known from practice, in which a normally electrically driven closed-loop compressor conveys the entire fluid flow, the integration of a jet pump that is also referred to as an ejector is associated with far lower investment costs. Although a motive fluid must be fed at a higher pressure within the scope of the method according to the invention, with respect to the entire process advantages result with regard to investment and operating costs, since usually a lower motive fluid gas flow is sufficient in order to achieve high fluid flow in the closed loop. Starting from the usual design of a process plant, the volumetric flow ratio of motive fluid to inlet fluid is between 1:20 and 1:8, preferably approximately 1:9. Within the scope of the method according to the invention the motive fluid can be held in a comparatively small is pressure tank.

In addition to heating the flow passages of a process plant, that is, starting up the initially cold process plant after an operating break, the method according to the invention can also be used to keep a process plant in an operational condition without impingement by a process fluid to be treated, while maintaining the necessary operating pressure and the operating temperature, which is also referred to as hot standby. A hot standby can be provided, for example, when the normal operation of the process plant is interrupted only briefly. In the case of several process plants arranged parallel to one another, individual plants can also be set temporarily at hot standby to adjust to a different task.

In particular with complex plants, which contain a plurality of stages, a hot standby can also be provided over a longer period of time, since particularly with plants of this type the resumption of normal operation from hot standby can take place much more quickly and easily than when individual stages of the overall plant have to be started up from a cold state.

When the method according to the invention is provided for keeping warm the flow passages of a process plant, it must be taken into consideration that the process plant has already reached the operating temperature. With a process control of this type, therefore, only the heat losses occurring during an interruption in operation must be compensated for, in that by means of the jet pump the pressure loss occurring during the movement of the fluid flow in the closed loop as well as by a corresponding preheating of the compressed fluid flow the heat losses are compensated for. Since furthermore a temperature increase in the flow passages is not necessary, the method according to the invention can be carried out with an overall smaller quantity of the motive fluid.

As already explained at the outset, the method according to the invention is provided in particular for process plants in which a catalytic conversion of the constituents takes place in at least one plant stage, since with catalytic reactions predetermined temperatures must be observed precisely. In particular the flow passages of the process plant can contain at least one apparatus with bulk solids.

In order within the scope of the present invention to achieve a heating or keeping warm of the flow passages in the process plant, the pressure as well as the volumetric flow ratio can be reduced to a certain extent compared to normal operation of the process plant. In order to achieve an effective heating or keeping warm of the flow passages, however, within the entire closed loop advantageously a pressure of more than 5 bar, preferably more than 10 bar, particularly preferably more than 25 bar is maintained. The volumetric flow of the fluid flow provided for heating or keeping warm the flow passages preferably corresponds to less than 15%, for example, between 5% and 10% of a nominal volumetric flow, for which the process plant is designed is with full work load in normal operation.

The fluid flow provided within the scope of the method according to the invention as well as the process gas flow provided in normal operation are usually present as gas and/or vapor. Thus an inert gas, preferably nitrogen or carbon dioxide, a fluid containing gaseous constituents and water vapor or also only water vapor can be the motive fluid of the jet pump.

With the operation of the jet pump by water vapor, according to a preferred further development before the return of the fluid flow to the jet pump water is condensed out and removed, the water quantity condensed out of the fluid flow corresponding to the water quantity that is fed to the jet pump as water vapor. With process control of this type, a balance can be achieved between the water added and the water discharged.

Alternatively, however, independently of the type of motive fluid fed to the jet pump, part of the fluid in the closed loop is extracted. This is necessary in particular, when taking into consideration added motive fluid, the total fluid flow fed in the closed loop to the flow passages of the process plant is to be kept constant, in that then the volumetric flow of the extracted part corresponds to the volumetric flow of the fed motive fluid. Furthermore, heat is recovered from the extracted part of the fluid by a heat exchanger in order to use the recovered heat elsewhere, for example, for preheating the motive fluid.

In order to heat the fluid flow within the scope of the present invention, a heat exchanger that is acted on with a heating medium, and/or an electrically operated heating element can be used as a heater. The arrangement of several, even differently designed, heating elements can be effective in particular when during a heating process a predetermined temperature range is to be maintained, temperature increases of differing strengths being realized over the time course in particular by an easily controllable electric heater.

When the process plant has several stages, it can be necessary or at least advantageous to heat them or keep them warm at different temperatures and heating loads. To this end according to a preferred embodiment of the invention the fluid flow guided in the closed loop is split in order to thus act on the at least two stages with different quantities of fluid. Within the scope of the invention then optimum conditions for heating or keeping warm can be adjusted for each stage. In order to make possible a splitting of the fluid flow, a flow splitter can be provided downstream in the flow direction of the jet pump and/or in the return line to the jet pump, which flow splitter contains a flow controller in the form of fittings or valves.

The invention is described below with reference to a drawing showing only one embodiment. Therein:

FIG. 1 shows a process plant in auxiliary devices can heat up or keep warm the flow passages outside normal operation;

FIG. 2 shows a modified further development of the process plant shown in FIG. 1.

FIG. 1 shows a process plant in which during normal operation a process gas is fed in through a supply line 1 and is subjected in various stages 2 a and 2 b to a chemical and/or physical treatment before it leaves the process plant through an output line 3. At least one of the stages 2 a and 2 b acts as a catalyst that preferably contains a bulk material as catalytic agent. Additionally or alternatively, however, catalysts with catalytic mats or other separation or conversion devices can also be provided.

FIG. 2 shows an embodiment of a process plant having two stages 2 a and 2 b that are heated and/or kept warm in a different manner. With an otherwise comparable structure, to this end seen in the flow direction downstream of a jet pump 5 a flow splitter is provided where the fluid flow can be diverted in variable proportions by a first fitting 11 a to the first stage 2 a and by a second fitting 11 b to the second stage 2 b. In addition in a recovery line 4 a further flow splitter with additional fittings 12 a and 12 b is provided. Due to the complete or partial opening and closing of the fittings 11 a, 11 b, 12 a, 12 b the fluid flow can be divided differently among the two stages according to requirements. If, for example, the fittings 11 a and 12 b are opened and the fittings 11 b and 12 a are closed, the entire fluid flow is conveyed according to FIG. 1 through both stages 2 a and 2 b. If, however, the fittings 11 a and 12 a are opened and the fittings 11 b and 12 b are closed, the entire fluid flow is guided only through the first stage 2 a. A closed loop through the stage 2 b is achieved when the fittings 11 b and 12 b are opened and the fittings 11 a and 12 a are closed. As already previously explained, a freely adjustable ratio of the partial flows conveyed over the first stage 2 a and the second stage 2 b can be adjusted by the degree of opening of the individual fittings.

The present invention relates to a method of heating or keeping warm the flow passages of the process plant outside normal operation. To this end the return line 4 serves to direct fluid flow in a closed loop. To compensate for the pressure loss occurring in the closed loop, the fluid flow is fed to a jet pump 5 in the closed loop and there is acted on with a motive fluid fed at a higher pressure by a nozzle 6 to the jet pump 5. The motive fluid fed from a pressure tank 7 mixes with the inlet fluid in the jet pump 5. In order to be able to heat up or keep warm the flow passages of the process plant, furthermore a heater 8 is provided, which in the illustrated embodiment shown is a heat exchanger. As an additional or alternative heater among other things an electrical heating element can also be provided.

When feeding in an inert gas such as nitrogen or carbon dioxide as motive fluid to the jet pump 5 or feeding in a motive fluid that contains gaseous constituents and water vapor, part of the fluid guided in the closed loop through an output line 9 is extracted so that taking into consideration the added motive fluid the entire fluid volume remains constant or at least within a predetermined range. Advantageously, the extracted part is directed for heat recovery through a heat exchanger 10. Furthermore, it is possible to feed water vapor as motive fluid to the water jet pump 5. With an embodiment of this type, part of the fluid flow can be extracted as a gas or vapor. Furthermore, however, there is alternatively the possibility of cooling the fluid flow before returning it to the jet pump 5 so that water is condensed out of and extracted from the fluid flow. There is the possibility thereby of making the quantity of water condensed out of the fluid flow the same as the quantity of water that is fed as water vapor to the jet pump 5. The additional extraction of gas or vapor is not necessary within the scope of a process control of this type.

According to a concrete illustrated embodiment, nitrogen, carbon dioxide or water vapor is held in the pressure tank 7 as a reservoir at a high pressure of approximately 60 bar and fed as a motive fluid to the jet pump 5 so that the fluid flow passing in the closed loop at a pressure of approximately 20 bar undergoes a pressure increase that compensates for the pressure loss occurring in the closed loop. With heating, a relatively small pressure loss must be overcome, since only a fraction of 5% to 10% of the nominal volumetric flow provided for normal operation at full load is necessary. This results in a low compression ratio, so that with a low motive fluid gas flow a comparatively large fluid flow can be circulated in the closed loop. If, for example, at an operating pressure of 35 bar the total pressure loss in normal operation is 3 bar, during heating at a reduced pressure of only 20 bar, a pressure loss of only approximately 0.1 bar is to be expected. At the jet pump 5 the ratio of motive fluid to inlet fluid is then about 1:9.

According to a further concrete illustrated embodiment the process plant is heated with water vapor as motive fluid for the jet pump 5 so that due to the selection of the starting pressure and the circulating quantity it is possible even at relatively low temperatures of typically 100° C. in the bulk material to render possible a heating of the process plant without condensation of water vapor on the occurring in the bulk material, which generally must be prevented. In order to ensure this, a low starting pressure is adjusted and a cooler is provided in the closed loop of the process plant to condense out a large part of the water contained in the fluid flow as water vapor. In this manner, despite the use of water vapor as motive fluid, the dew point of the water vapor can be kept below the temperature prevailing in the catalyst bulk material. 

1. A method of heating or keeping warm the flow passages of a process plant for the catalytic conversion of constituents of a process fluid outside the process operation during which the process fluid flows into the plant and there is subjected to a chemical or physical treatment or conversion, a fluid flow provided instead of the process fluid outside the process operation for heating or keeping warm the flow passages being guided in a closed loop containing a heater for warming up the fluid flow, wherein to compensate for the pressure loss occurring in the closed loop the fluid flow is fed as an inlet fluid to a jet pump in the closed loop and there is acted on with a motive fluid fed at a higher pressure.
 2. (canceled)
 3. The method according to claim 1, wherein flow passages of the process plant contain at least one apparatus holding bulk solids.
 4. The method according to claim 1, wherein a pressure within the entire closed loop is greater than 5 bar.
 5. The method according to claim 1, wherein the volumetric flow of the fluid flow provided for heating or keeping warm the flow is less than 15% of a nominal volumetric flow for which the process plant is designed with full work load in normal operation.
 6. The method according to claim 1, wherein an inert gas, preferably nitrogen or carbon dioxide, is fed to the jet pump as a motive fluid.
 7. The method according to claim 1, wherein a fluid containing gaseous constituents and water vapor is fed as motive fluid to the jet pump.
 8. The method according claim 1, wherein water vapor is fed as motive fluid to the jet pump.
 9. The method according to claim 8, wherein before return of the fluid flow to the jet pump, water is condensed out and removed from the fluid flow, the quantity of water condensed out of the fluid flow corresponding to the quantity of water fed to the jet pump as water vapor as motive fluid.
 10. The method according to claim 1, wherein part of the fluid guided in the closed loop is extracted.
 11. The method according to claim 10, wherein a volumetric flow of the extracted part corresponds to the volumetric flow of the fed-in motive fluid.
 12. The method according to claim 10, wherein the extracted part of the fluid is guided for heat recovery via a heat exchanger, the motive fluid being preheated with the recovered heat.
 13. The method according to claim 1, wherein a heat exchanger that is acted on with a heating medium is used as a heater for heating the fluid flow.
 14. The method according to claim 1, wherein an electrically operated heating element is used as a heater for heating the fluid flow.
 15. The method according to claim 1, wherein the motive fluid is held in a pressure tank.
 16. The method according to claim 1, wherein the process plant has at least two stages and that a flow splitter containing a flow controller is provided seen in the flow direction behind the jet pump, flow splitter renders possible a variable division of the fluid flow between the first stage and the second stage.
 17. A method of operating a process plant for the catalytic conversion of constituents of a process fluid where during normal operation of the plant the process fluid is fed into the plant, then passed through flow passages and flowed in a treatment stage over a bulk catalyst, and thereafter fed out of the plant, the method comprising, for heating or keeping warm the flow passages outside of normal operation, the steps of: providing a recovery line forming with the flow passages a closed loop passing through the treatment stage, through a heater, and from an inlet to an outlet of a jet pump having a motive-fluid nozzle; and supplying a motive fluid under higher pressure than the process fluid in the closed loop to the nozzle so as to circulate the process fluid through the closed loop such that heat picked up from the heater is transferred to the flow passages and bulk material in the treatment stage. 